Fibre optic transmitters and transceivers are widely used in data-communication and telecommunications applications. In a typical optical signal transmitter, a light source, for example, a laser source, is biased at an average optical power point with a constant bias current. To encode data for transmission, the bias current is usually modulated around this bias point. For most optical communication applications, a binary modulation corresponding to a logic high “1” or logic low “0” is commonly used. Typically, signal information is transmitted by stream of pulses comprising “On” and “Off” pulses or “Hi” or “Lo” pulses. For example, an “On” pulse is sent with a higher modulation current and an “Off” pulse is sent with a lower modulation current or vice versa. For good transmission performance, the output optical power of the “Off” pulse should be as low as possible and it is ideal if there is no optical output when an “Off” pulse is being transmitted to obtain a good “eye-diagram” with a wide “eye-opening”. To attain this purpose, the laser current should be as low as possible when an “Off” pulse is sent. On the other hand, because the laser driving current must stay above the threshold current in order to attain a good dynamic response, the lowest laser current corresponding to an “Off” pulse should be as near the threshold current as possible, whereby a balance between a good dynamic response for high speed operation and a good extinction ratio can be struck. The extinction ratio is a ratio between the high and low level signal of an optical output power and a good extinction ratio is typically represented by an eye-diagram with a wide opening. Thus, control of laser current is critical for good performance of an optical transmission system.
Vertical-cavity surface-emitting lasers (VCSELs) are extensively used in fibre optic communications, especially for data-communication applications. A typical optical output power versus current characteristics of an exemplary light source comprising a VCSEL is shown in FIG. 1. For a typical laser source, the threshold current and differential quantum efficiency change with temperature, although the specific change of threshold current depends on the device designed. Generally speaking, the threshold current of a laser source has a minimum at around room temperature, the threshold current increases with higher and lower operating temperatures. The quantum efficiency normally decreases with increasing temperature.
Modulation of an optical source is typically by a modulation driver. MAX 3930, MAX 3701, MAX 3735A and MAX 3740A from Maxim are exemplary modulation drivers for providing modulation current to a laser source for signal transmissions. However, conventional modulation drivers are not satisfactory for operation above room temperatures, for example, above 70° C. and are not satisfactory for operation within a wide temperature range, for example, from room temperature to 125° C.